ct

1. COMPUTED
TOMOGRAPHY
BASIC PRINCIPLES

2. TOMOGRAPHY
• Also known as Computerized Axial Tomography (CAT Scan)
• Comes from the Greek word Tomos- to cut
• Preliminary image- Topogram (Siemens), Scanogram (Toshiba), Scout (G.E)

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4. CT IMAGE
QUALITY:CRITERIA
Spatial Resolution- ability of a system to define small
objects distinctly
Low Contrast Resolution-ability of a system to
differentiate, on the image, objects with similar densities
Temporal Resolution- speed that the data can be acquired

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X axis- width
Y axis- height
Z axis- thickness of the slices
Pixel-two dimensional squares
Picture element(X & Y)
Voxel- three dimensional cube
Voxel element (X, Y & Z)
Matrix-grid formed by rows and columns
512 x 512- common matrix size in CT
Total of 262,144 pixels

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Photons- bundle of energy in
an xray beam
Attenuation- the degree to which
an xray beam is reduced by an
object
The quantities of xray photons that pass
through the body determines the shades of
gray on the image.

7. The number of photons that interact
increases with density, thickness &
atomic number
Low attenuation- xray photons that pass through
objects unimpeded
-black area on image
High attenuation- object that has the ability to
absorb much of xray beam

8. Density- defined as the mass of the substance
per unit volume.
To differentiate adjacent objects on a CT image,
there must be density difference between two
objects.
Measurements are expressed in Hounsfield units (HU)

9. The amount of xray beam that is scattered or
absorbed per unit of the absorber is expressed
by Linear attenuation coefficient
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10. High density contrast agents/ Positive
agents- barium sulphate, iodine (Scanlux,
Omnipaque, Visipaque)
Low density CA/ Negative agents- water

11. Sir Godfrey N. Hounsfield, DSc
Father of computed tomography
Won a nobel prize in 1979 with
Allan M. Cormack
Hounsfield units-aka CT numbers/ density values.
Quantify the degree that a structure attenuates
an xray beam
Distilled water- 0
Bone- +1000
Air- -1000
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12. All xray beam sources for CT and radiography
produce xray energy that is polychromatic
Artifacts are object seen
on the image but not present
in the object scanned. It
degrades image quality
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13. Beam hardening artifact/ Cupping
Artifact- most common when xray
Beam first penetrates a dense
Structure. Appears as dark streaks

14. Volume Averaging- The process in CT by which different
tissue attenuation values are averaged to produce one
less accurate pixel
Partial volume artifact occurs when tissues of widely
different absorption are averaged together in a slice.
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Can be minimised
by creating thinner
slices of the same area

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16. Scan Modes
Step and Shoot Scanning- (1980s) aka axial
scanning/ conventional scanning/ serial
scanning
Helical/ Spiral Scanning
Multidetector Row CT Scanning

17. STEP AND SHOOT SCANNING
Advantage:
Highest image quality, superior than spiral/helical methods
Disadvantage:
Longer scan time
Misregistration-esp when there is motion

18. HELICAL SCANNING/VOLUMETRIC SCANNING
Continually rotating xray tube, constant xray output, uninterrupted
Table movement
Uses slips rings for continuous rotation of xray tube
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19. Pitch-travel distance of CT
scan table per 360 rotation of
the xray tube divided by xray
beam collimation width
Pitch of 1 or 1.5 is common
Higher pitch-Faster table
movement-decrease scan time-
increase image noise
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20. SINGLE DETECTOR CT/MULTIDETECTOR CT
SDCT MDCT
Single gantry rotation can
produce multiple slices

21. CT PROCESS
DATA ACQUISITION
IMAGE RECONSTRUCTION
IMAGE DISPLAY
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22. 1. GANTRY
DATA ACQUISITION
70-90 cm Aperture
+/-15 degrees to +/-30 degrees tilt
Forward and backward tilt
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23. • SLIP RINGS
-Brush like apparatus to provide continuous, electrical
Power and electronic communication across a rotating
surface.
-used in helical scanners
• GENERATOR
-High frequency
-Power capacity in KW
-120-140 kV
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24. • XRAY TUBE
Tungsten anode
Focal Spot size 0.5 and 1.0
• FILTRATION
Compensating filters-shape the xray beam
Ex: Bow tie filter
Reduce the radiation dose by filtering the long wav
• COLLIMATION
-restrict the xray beam reducing scatter radiation
-controls slice thickness

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26. Types of collimator
Source collimator/Pre-patient collimation
Located near the xray source
Affects patient dose
Determines how the dose is distributed across the slice
Thickness
Resembles small shutters, dependent on operators
Selection of slice thickness
Predetector collimator/Postpatient collimation
Ensure the beam is the proper width
Prevent scatter radiation from reaching the detector

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30. • DETECTORS
Optimal Characteristics:
1. High detector efficiency- ability to capture transmitted photo
2. low/no afteglow- brief persistent flash of scintillation
3. High scatter suppresion
4. High stability

31. Factors of Detector Efficiency
1. Stopping power of the detector material
2. Scintillator efficient (solid state)
3. Charge collection efficiency (xenon detectors)
4. Geometric efficiency- amount of space occupied by
the detector collimator plates relative to the surface
area of the detector
5. Scatter rejection
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32. Capture Efficiency- ability which detector obtains
photons that have passed through the patient
Absorption efficiency- number of photons
absorbed by the detector and is dependent on the
physical properties of the detector face (thickness,
material)
Response time- time required for the signal from
the detector to return to zero after stimulation by
xray radiation, so that it is ready to detect another
xray event.
Dynamic range- ratio of maximum signal
measured to the minimum signal the detectors can
measure.

33. TYPES OF DETECTORS
XENON GAS DETECTORS
(GAS FILLED ION CHAMBER)
SOLID STATE CRYSTAL
DETECTOR
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34. XENON GAS DETECTORS
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Less expensive
Highly stable
Consist of three tungsten plates
Has aluminum casing
(kept under pressure)
Photon->ionises xenon->ions are
amplified->collected charge
produces current->raw data
Capture about 50% photons
in the beam

35. SOLID STATE CRYSTAL DETECTOR
A.k.a. Scintillation Detectors
Use crystal that fluoresces when struck by xray
photon.
Cadmium tungstate, bismuth germanate,
Cesium Iodide, Ceramic rare earth compounds
(Gd & Yttrium)
Photodiode- converts light energy
to electrical energy
About 80%
capture efficient

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37. SCANNER GENERATION
Produced by EMI medical division
Scan time-4-5mins, Matrix 80x80, 180times repeat linear motion
In this unit head was enclosed in a water bath
Rotate-Translate,Pencil Beam

38. Detector array(30) was incorporated
Scan time: 10-90 sec
Rotate-Translate, Narrow Fan beam

39. Fixed relationship between Xray source and detectors(arc)
A.k.a Rotate-rotate scanner
Disadvantage: Ring Artifacts, Scan time 2-10 sec
Rotate-rotate wide fan beam

40. Fixed detector array(ring)
A.k.a Rotate only system
Rotate-Stationary

41. EBCT/ ELECTRON BEAM IMAGING/ULTRAFAST CT
Produced by Imatron, uses a large electron gun as an xray source
Neither the xray beam or detectors move
Scan can be acquired in shorter time (acquisition time-100msec)
For cardiac and paediatric imaging
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46. • DAS/ DATA ACQUISITION SYSTEM
Measures the number of
photons that strikes the
detector, converts the
information to a digital
signal and sends the
signal to the computer

47. 2. PATIENT TABLE
Incrementation- process of
moving the table by a
specified measure. Also
referred to as feed, step or
index.

48. CT PROCESS
DATA ACQUISITION
IMAGE RECONSTRUCTION
IMAGE DISPLAY

49. IMAGE RECONSTRUCTION
Algorithm-finite set of unambiguous steps performed in a
prescribed sequence to solve a problem.
Fourier transform- developed by Baron Jean Baptiste Joseph
Fourier (17th century)
-method to study waves of many different sorts and solve kinds
of linear differential equations
Interpolation- mathematical method of estimating a value of an
unknown function using the known value on either side of the
function. (creating a missing data)
TERMINOLOGY

50. DATA TYPES
Raw Data
Refer to the fatal sitting in the computer waiting to be made into
an image. The process of using raw data to create an image is
called Image Reconstruction
Prospective reconstruction- automatically produced during
scanning
Retrospective reconstruction- uses raw data to generate
new image after scanning

51. Image Data
Are those which result once the computer has processed
the raw data. One hounsfield unit value is assigned to
each pixel
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56. FILTERED BACK PROJECTION
Filtering is applied to the scan data BEFORE back
projection occurs.
Convolution-applying a filter function to an attenuation
profile
Filtered back projection algorithms use Fourier theory to
reduce statistical noise

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59. ADAPTIVE STATISTICAL ITERATIVE RECONSTRUCTION
Use large variety of algorithms
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60. Scan Field of View (SFOV)
Calibration field of view.
Determines the area from which
the raw data are acquired
Small SFOV- 25 cm (head)
Medium-35 cm
Large 42-50 cm
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61. Display Field of View (DFOV)
Zoom/ Target
Determines how much of the
collected raw data is used to
create an image
Cannot be larger than SFOV
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62. CT PROCESS
DATA ACQUISITION
IMAGE RECONSTRUCTION
IMAGE DISPLAY

63. IMAGE DISPLAY
WINDOW WIDTH
s the quantity of hounsfield units represented as shades of grey on a sp
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WINDOW LEVEL
Selects which hounsfield values are displayed as shades of gray.
Selects the centre CT value of the window width

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Wide window width (500-
2000 HU)- best for imaging
tissue types that cary
greatly
Narrow window width (50-
500HU)- tissues with similar
densities

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